In recent years, a method for forming a tire using a rigid core (a) including a core body (a1) having a tire forming surface (as) on an outer surface thereof as shown in FIG. 10 (hereinafter generally referred to as “core method”) has been proposed to enhance accuracy in forming a pneumatic tire (refer to, for example, patent document 1).
With the core method, tire structural members, such as a carcass ply, a belt ply, a sidewall rubber, and a tread rubber, are sequentially stuck one upon another on the tire forming surface (as) so as to form a green tire (t) having approximately the same shape as a finished tire (corresponding to a tire product after being subjected to vulcanization). The green tire (t), together with the rigid core (a), is then loaded into a vulcanization mold (b) so as to subject the green tire (t) to vulcanization molding between the core body (a1) as an inner mold and the vulcanization mold (b) as an outer mold.
On this occasion, the core body (a1) is heated in the vulcanization mold (b) from a low temperature state of 100° C. or below to a high temperature state of approximately 150° C. in order to subject the green tire (t) to heat vulcanization. Therefore, thermal expansion occurs in the core body (a1), and a shape dimension of the core body (a1) varies in the vulcanization mold (b).
As conceptually shown in FIG. 11(A), thermal expansion in a cross-sectional direction ε1 and thermal expansion in a radial direction ε2 occur as the thermal expansion. The thermal expansion in the cross-sectional direction ε1 is the thermal expansion that causes the outer surface (as) to expand so as to swell outward in a meridional cross section of the core body (a1). The thermal expansion in the radial direction ε2 is the thermal expansion that causes the ring-shaped core body (a1) to expand so as to increase a diameter thereof outwardly in a tire radial direction around a tire axis (i). consequently, as conceptually shown in FIG. 11(B), expansion (d) varies across the cross section of the core body (a1). In particular, the thermal expansion in the radial direction ε2 is added to the thermal expansion in the cross-sectional direction ε1 so as to increase the expansion (d) on a side closer to a tread than a maximum width position P0 of the tire forming surface (as) (namely, on a radially outside). In contrast, the thermal expansion in the radial direction ε2 is subtracted from the thermal expansion in the cross-sectional direction ε1 so as to decrease the expansion (d) on a side closer to a bead than the maximum width position P0 (namely, on a radially inside).
The vulcanization mold (b) is controlled at a constant temperature (for example, approximately 160° C.) over the whole vulcanization process. Therefore, the shape dimension of a cavity surface (bs) of the vulcanization mold (b) remains almost unchanged. The nonuniformity of the expansion (d) of the core body (a1) causes nonuniformity of vulcanizing pressure applied to the green tire (t) (pressing force applied to the cavity surface (bs)). That is, the vulcanizing pressure becomes high on the tread side and low on the bead side. This leads to the problem that the quality of the tire is deteriorated due to insufficient vulcanization on the bead side subjected to the low vulcanizing pressure, and due to difficulties in discharging air remaining within the tire.